This application claims the priority of U.S. Provisional Application No. 61/224,171 filed Jul. 9, 2009, the contents of which are hereby incorporated by reference.
Liposomes and Water-in-Oil-in-Water (W/O/W) emulsions are typical known examples of water-soluble drug delivering carriers. However, these carriers are problematic because they involve drug leakage and difficulties in controlling the particle size.
Meanwhile, along with the developments in biotechnology, there have been active attempts to deliver various substances into a target cell, including peptides involved in the cellular signal transduction system, proteins having physiologically important roles in the cell, and expression genes for these products. It is often the case that the expression genes are more suited for delivery into the cell than the product proteins. The reasons for this include the simpler structure (genes do not have as large differences as proteins in overall structures and properties even when the proteins they express are different), good expectancy for lasting efficacy after the introduction into the cell, and no risk posed by the expression genes themselves, even though the expressed proteins may involve a risk or a pathogenic factor. The genes used for this purpose may be genes administered to cause expression of proteins lacking in a diseased cell, or genes that can induce cell death, such as caspase and thymidine kinase expression genes. There have been attempts to use genes themselves as drugs. It is therefore believed that the development of gene vectors having high expression efficiency (gene carriers with high gene introducibility into a cell and high expressibility) or control systems for such gene vectors have important roles in future medical developments (Non-Patent Document 1).
Administration of a gene alone into a cell produces only marginal expression efficiency. This is known to be primarily due to the high anionic properties of the cell membrane structure that includes sugar chains such as proteoglycan. The DNA molecule, as the building block of genes, is a polymer that also has anionic properties. At present, virus vectors produced by inserting or replacing a therapeutic gene into the gene sequence of viruses such as adenovirus and retrovirus represent the most efficient gene vectors. The virus vector takes advantage of the inherent characteristics of the viruses carrying their own genes into a cell, and can achieve high-efficient gene expression; however, its development is limited by safety and productivity (Non-Patent Document 2). As a countermeasure, there have been active studies to create non-viral gene vectors, as represented by, for example, cationic liposome. The commercially available Lipofectamine and Lipofectin, in particular, have become the most mainstream non-viral gene vectors, because these gene vectors have high gene introduction efficiency, and are highly useful and reproducible for many types of cells (Non-Patent Document 3). Aside from these examples, it is known that cationic polymers such as polyethyleneimine (Non-Patent Document 4) and chitosan (Non-Patent Document 5), and amino acid dendrimers (Non-Patent Document 6) also can be used for gene expression. All of these examples are based on the concept that the genes having anionic properties form a complex with the cationic molecules and break through the negatively charged cell membrane. However, these techniques, by themselves, lack serum resistance and blood stability, pose difficulties for use in vivo, and are often limited for further functionalization. There have been attempts to construct a DNA-encapsulated capsule carrier for further functionalization, as described in Non-Patent Document 7 (Multifunctional Envelope-type Nano Device; MEND), and Non-Patent Documents 8 and 9 (polymer micelle). However, these techniques all use cationic molecules for gene encapsulation, and have drawbacks that controlling the expression of the introduced gene in a cell is difficult, and that the techniques are only applicable to the encapsulation of substances having high anionic properties.